Audio frequency amplifier



P 1941- J. M. RIDDLIVE, JR 2,255,679

AUDIO FREQUENCY AMPLIFIER Filed May :51, 1939 2 a a 5a m 9 /'0// 12 Wflrra 0073 07 Patented Sept. 9, 1941 AUDIO FREQUENCY AIVIPLIFIER,

James M. Riddle, Jr., West Collingswood, N. J., assignor to Radio Corporation of America a corporation of Delaware Application May 31, 1939, Serial No. 276,680

2 Claims. (01. 179171) 1 The present invention relates to audio, frequency amplifiers, and has for its object to provide an improved audio frequency amplifier for producing large power output without distortion, with a minimum number of amplifier tubes and with a relatively low power audio frequency input voltage.

The invention is particularly adapted for use in battery operated radio receivers wherein the power supply is limited, while, at the same time, a relatively high undistorted power output is desired.

In providing a high power output, it is desirable to utilize a class B or class AB audio frequency output amplifier, requiring power over all or part of each cycle, and it is therefore a further object of the present invention to provide an audio frequency amplifier having adequate driving power and suflicient gain to operate with an output stage requiring driving power from a high impedance source, such as a high impedance triode amplifier, and with a small tube complement.

It is also a further object of the present invention to provide an improved audio frequency amplifier for low power radio apparatus providing a relatively high gain, and high power output with low distortion through the effective use of inverse or degenerative feedback.

It is also an object of the present invention to provide an improved audio frequency amplifier having a class B output power stage of the zero bias type requiring driving power over substantially the full frequency cycle, with a relatively high gain driver stage therefor of the pentode type permitting a high order of inverse feedback to overcome distortion in the feedback loop, including the driver stage.

In battery operated receivers and amplifiers, the class B or class AB output stage is advandesired value such as is normally provided without inverse feedback.

The invention will, however, be better understoodfrom the following description when considered in connection with the accompanying drawing.

'high impedance triode, voltage amplifier provided by a tube 5 as a first stage having a control grid 6 connected to the volume control contact I of a volume control potentiometer 8, to which incoming audio frequency signals are applied through an input lead 9 and a ground or chassis tageous in that in the absence of received signals the anode current is relatively low. A disadvantage of the class B or class AB amplifier stage lies in the fact that it requires a relatively high powered driver. While inverse feedback tends to decrease distortion in an amplifier, it also tends to reduce the over-all gain of the amplifier.

Accordingly, it is also an object of the present invention to provide an improved audio frequency amplifier which permits a class B or class AB amplifier stage to be utilized in connection with a high gain driver stage which permits a high order of inverse feedback without appreciably reducing the overall gain below a5;

connection It.

The high impedance first stage amplifier tube may be included in the same envelope with diode detector anodes indicated at H having a common cathode I2 connected to ground or chassis I3 through a low resistance bias resistor M. The bias potential from the resistor M is applied to the control grid 6 through a grid resistor l5 connected to chassis as indicated at It.

The amplifier output stage comprises a pair of class B amplifier triodes 20 and 2| having a balanced input circuit comprising the secondary 22 of a step-down transformer 23 providing a low resistance input circuit connected at its terminals to the control grids 24 and having a center tap 25 connected to ground 25 and to the cathodes 27, as indicated, for zero bias class B operation. The output anodes 28 are connected in balanced relation to the primary 29 of the class B output transformer 30, the secondary 3| of which is connected with the loudspeaker load represented by the voice coil 32.

One side of the secondary circuit is grounded to chassis as indicated at 34. The opposite high potential side of the secondary is provided with a terminal connection 35 for an inverse feedback lead 36 having a series controlling resistor 31 therein and an output connection to the cathode terminal 38 of the cathode resistor [4 in the first -stage, thereby to provide inverse feedback to the first audio frequency stage under control of the resistor 31. '7 1 The amplifier utilizes a pentode tube 40 as the second stage amplifier or driver stage for the into the feedback circuit. y I v It has been found that, with a resistanceof output stage. The use of a pentode tube for a driver stage ordinarily results in severe distortion and an unsatisfactory fidelity characteristic.

However, the pentode driver has the advantage 7 7 that a higher power output is obtainable therefirst audio frequency amplifier stage 25.

The advantage of the high signal gain and power of ,the pentode is retained by providing a relatively high degree of inverse feedback-which is sufficient to correct the high-distortion characteristic of the pentode driver stage, while a portion of the relatively high gain is maintained through the amplifier substantially higher'than available witha usual triode driver. i

In the preferred arrangement shown, the pentode driver stage "is connected between the first stage tube and the class -B stage 20'2l by connecting the output anode of 'thepentode to a high impedance primary 4 of an interstage coupling transformer 23, and the-input or signal grid 44 is coupled to the high impedance output circuit 45 of the first stage amplifier through the usual capacity; coupling means 45 provided with a high resistance grid leak 47.

The pentode is provided with an unb-ypassed cathode resistor 48 from which bias for the control grid M'is derived'an-d a predetermined degree of degeneration which further aids'in'preventing distortion. Furthermore, the 'unbypassed resistor does not introduce any phase shift. The

cathode circuit is grounded'as indicated at '53.

The suppressor grid 5f is connected to ground and-thescreengrid'iZ is connected to the positive-supply lead indicated at 53, which also'supplies the anode ii.

plifier, an RCA GJ'Tpentode as the tube 48 and the tubes .29 and 21 provided by a single 'RCA'SN? class B twin output tube, highly stable amplifier operation has been obtained-in a lo-w'power battery amplifier with a resistor of substantially .5

me'gohm' at 45 and Gil, a coupling capacitor of. .5 mm'faat itk and a grid leak'of -l: megohm at 1'5.

The self-bias resistor t8 without by-pass is approximately 600-ohmsl The resistor Min the cathode circuit of the first stage amplifier -5 ispreferably of, a low value and may be of the order of, 680 ohms 'to prevent degeneration the first stage 5. For this reason,'bleeder current is provided therethrough from a suitable source controlled by a series resistor indicated at 53. The Icy-pass capacitor 69 is of avalue sufiiciently small to prevent introducing any frequency characteristic 1500 ohms, at 37 a feedback of the order of 32* db. isfpermissible, and that when this resistance 'is' increased to 3200; chars and the feedback is inereased to substantialiy 36 -db., stable operation is possible, for the reason that thepento'de driver stage is included "in the feedback jloopbietween V the amplifier input circuit and theoutput stage;

Heretofore, (as ispwellknown. Jan finverse feedb'ack This amplifier is highly effective and economical for battery operated receivers and the like 1 for the reason that a high gain tube such as a pentode may be used as a driver stage for a class B amplifier. The high power sensitivity makes it possible to drive the class B or class AB output stage effectively without appreciably loading a high mu or high impedance first stage amplifier. The distortion of the pentode may entirely be eliminated by the high order of feedback through the loop including the pentode tube while the remaining gain of the pentode is efiective to provide a desired over-all gain for the amplifier.

For a given wattage output, the three-stage amplifier shown has an over-all characteristio as indicated by the curve 65 in Fig. 2, whereas, with obtained with T8, conventional class B ieedback amplifier provided with a triode driver stage-and first stage amplifiera Summarizing the foregoing, the inversefeedback loop is provided through the three amplifier stages, thereby correcting distortion and stage having a low gain and a relatively high power output, means for compensating for said low gain and for providing power for said input 7 circuit comprising an amplifier stage havinga f the order'of 15' db. has been considered" normal. 76

high power'sensitivity and a. high impedance output. circuit, a step-down transformer having a high impedance primary winding connected in, said high impedance output circuit and having a low impedance secondary connected in said first-named input circuit thereby to provide a step-down coupling connection between said high impedance output circuit a-nd said low impedance input circuit, an output circuit for said output stage, an output transformer having 'a primary connected in said last-named outputcircuit and having a low impedance secondary, means providing a loadcircuit connection for said secondary, mean-s' providing an inverse feedback loop through said amplifier stage and said low im pedance secondary to-compensate distortion genf erated'insai-damplifier stage,.and means for con.-

troll-ing the inverse feedback current through saidloop. 7 r

2. In an audio frequency amplifierhaving a zero bias, class B, balanced output stage requiring power input over at least a'. portion of each signal cycle, of m ean's 'for compensating the low gain of said amplifier stage with a minimum number of preamplifier stages comprising an am--v plifierst ge having a high power sensitivity and a. high impedance output circuit, a step-down transformer having a relatively high impedance primary connected with said last-named output circuit and having a relatively low impedance secondary connected in the input circuit of said output stage, a step-down output transformer for said output stage having a low impedance secondary, means providing an inverse feedback connection from said last-named secondary through said amplifier stage, and means in said feedback connection and in said amplifier stage for controlling the tendency of said amplifier to oscillate above the audio frequency range of 5 the amplifier when the feedback is intensified above a predetermined normal value.

JAMES M. RIDDLE, JR. 

